A theoretically-ideal bandpass filter passes any energy having a frequency within a desired frequency band and blocks any energy having a frequency outside the desired band. For many reasons, theoretically ideal filters cannot be achieved in practice. A good, practically realizable filter is generally considered to be a filter having a response that is essentially flat throughout the desired pass band and that smoothly drops off with either increasing or decreasing frequency outside the pass band. The response of a good realizable bandpass filter is shown in FIG. 1.
An acousto-optic filter ("AOTF") is an electronically tunable optical bandpass filter. Electronically tunable optical filters have been constructed so that an incident light beam of a first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first polarization to a second polarization of the light beam for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal. Bulk AOTF's fabricated in bulk crystals and using bulk acoustic waves and unguided optical beams have already found many important applications in laser and optics systems. Integrated AOTFs in which light is confined to a waveguide and which use surface acoustic waves are also expected to find important applications in laser and fiber optics systems, especially such as are used in modern telecommunications applications.
As shown in FIG. 2, an integrated AOTF is fabricated in an elongated crystalline substrate 30 such as lithium niobate (LiNbO.sub.3). An optical waveguide 34 is formed in an upper surface of the substrate 30, for example by indiffusion of titanium. A beam of light is coupled into the waveguide 34 through an input optical fiber 37. The light/propagates through the waveguide 34 and out through an output optical fiber 39. A surface acoustic wave beam is induced in the waveguide by an interdigitated transducer 32. The transducer is driven by an externally-generated electrical signal from a signal source 35. The frequency of the acoustic wave beam is determined by the frequency of the electrical signal. A pair of acoustic absorbers 38 absorbs acoustic energy produced by the transducer 34.
The acoustic wave beam induces a refractive index grating in the waveguide. The grating couples the transverse electric and transverse magnetic polarization modes of the light, but only within a narrow band of optical wavelengths. Thus, within this narrow band all the light propagating in one polarization mode is converted to the orthogonal mode, whereas outside this band the polarization mode of the light is unaffected
A first polarizer 41 adjacent the first extremity of the waveguide 34 blocks any incoming light that is not in a first polarization mode. Thus, only light polarized in the first mode is admitted to the filter. As the light travels through the waveguide, the polarization mode of any light having a wavelength within the narrow band of optical wavelengths is converted to a second mode which is orthogonal to the first mode. The polarization of the rest of the light is unaffected. A second polarizer 43 opposite the first polarizer 41 blocks from the output any light that is not in the second polarization mode. Thus, only light having a polarization mode that has been converted while passing through the filter is allowed to exit the filter. No output destination is shown, but it will be understood that the output light is ultimately provided to a user or to an optical device of some type.
The AOTF passes light having a wavelength within the band determined by the acoustic wave beam and blocks other light. Thus the AOTF serves as a bandpass filter. The center frequency of the pass band can be tuned by changing the frequency of the electrical signal that drives the transducer.
Unfortunately the frequency response of an AOTF is not like that of the good bandpass filter as shown in FIG. 1. Instead, the frequency response of an AOTF is characterized by unwanted sidelobes. In an ideal uniform single stage AOTF, sidelobes are generally less than 9.3 dB below the center frequency. A response curve of a typical AOTF showing the sidelobes is provided in FIG. 3.
A number of techniques have been proposed for sidelobe reduction. Cascading multiple sections has been proposed as one means of reducing sidelobes. If two AOTF section are used the sidelobes can be reduced to about 19 dB below the center frequency. The technique requires the inclusion of an integrated polarizer between the filter sections and results in a wider filter passband than a single filter AOTF of equivalent waveguide length.
Another method of reducing sidelobe levels is to vary the acoustic coupling along the length of the AOTF. A scheme involving directional couplers was described by Smith et al., Applied Physics, Vol. 61, page 1025 (1992), that results in a raised cosine acoustic interaction and is theoretically capable of reducing sidelobes to 17.5 dB below the center frequency per AOTF sectional. This technique has the disadvantage of increasing the 3 dB passband width by a factor of about 1.4 and increasing the acoustic power required by almost four times.
Finally, Trutna et al., Optical Letters, 18, page 1721 (1993), has described a technique for using controlled birefringence to vary phase mismatch as a function of length. This technique requires two AOTF sections and is capable of reducing sidelobes to 30 dB below the center frequency.
All of the above techniques require additional processing steps and require extra costs in the formation of the AOTF. It is desirable to be able to reduce the level of sidelobes in the AOTF frequency response without adding extra process steps to the formation of the AOTF.